Move DiffFileIndexView out into its own file.

This isn't used anywhere except the DiffViewerPageView, but neither are any of
the other views that make up the diff viewer, and the page view implementation
is a pretty big file.

Testing done: Smoke tested the diff viewer.
Reviewed at http://reviews.reviewboard.org/r/4642/
Add some very simple tests for the parse() methods in diffviewer models.

The diff viewer has several new models that aggregate bits of data required to
build the page. This change adds unit tests for the parse() methods, which
convert the names in the webapi into javascript-style camelCase names.

Testing done: Ran js-tests.
Reviewed at http://reviews.reviewboard.org/r/4643/